The present invention relates to a polyvinyl chloride-based resin composition or, more particularly, to a resin composition based on a polyvinyl chloride resin exhibiting excellent moldability with good flowability in the molding process and still capable of giving molded articles having excellent impact strength, tensile strength, heat resistance, weatherability and smoothness of surface.
As is well known, molded articles of a resin composition based on a polyvinyl chloride resin, referred to as PVC resin hereinafter, are imparted with increased properties such as tensile strength, heat resistance and the like as the average degree of polymerization of the PVC resin as the base ingredient of the resin composition is increased, in particular, in an unplasticized resin composition for rigid molded articles, of which the impact strength can be greatly improved even by compounding with a small amount of an impact-strength improver.
It is also known that a resin composition based on a PVC resin of a high degree of polymerization is disadvantageous in relation to the moldability. Namely, the flowability behavior of such a resin composition is necessarily poor while high flowability of the resin composition in molding is essential in any molding procedures including calendering, extrusion molding, injection molding, blow molding, vacuum forming and the like, especially when high-speed molding is undertaken so that the productivity of the molding process is greatly decreased and the molded articles have poor quality in respect of the low smoothness of the surface. A counter-measure for these problems accompanying the use of a high molecular weight PVC resin is to increase the molding temperature. However, extent of increase in the molding temperature is limited because the PVC resin per se is subject to thermal degradation. Therefore, the excellent properties inherent in PVC resins cannot be fully exhibited in the articles molded at such a high temperature when the average degree of polymerization of the PVC resin is so high that sufficiently good flowability of the resin composition without formulation of a plasticizer can be obtained only by increasing the temperature to such a level as to unavoidably cause thermal degradation of the PVC resin.
The problem of low flowability of the PVC-based resin composition is of course not involved in the molding process when the average degree of polymerization of the PVC resin is sufficiently low. However, the articles molded from such a resin composition are poor in impact strength which can be improved only by compounding the resin composition with an unduly increased amount of an impact strength improver such as a polybutadiene-based rubber resulting in a decrease in the heat resistance, tensile strength and other properties of the molded articles. It is also way possible way to improve the moldability of a PVC-based resin composition by compounding the composition with a processing aid or a lubricant though with disadvantages and problems on its part.
Accordingly, it is proposed to undertake the above mentioned various measures in combination so as to improve the moldability of the PVC-based resin composition. In particular, compounding of the resin composition with a polybutadiene-based rubber is effective for the improvement of the impact strength of the molded articles even with a relatively small amount of addition while admixture of a polybutadiene-based rubber causes another problem that the molded article of such a resin composition is subject to discoloration or a decrease in the impact strength and tensile strength after lapse of time because polybutadiene-based rubbers are susceptible to degradation by oxidation under the influences of heat and light.
On the other hand, Japanese Patent Kokai No. 50-88170 discloses a PVC-based resin composition comprising from 90 to 99 parts by weight of a PVC-based graft copolymer and from 1 to 10 parts by weight of a copolymeric resin of methyl methacrylate and/or acrylonitrile, styrene and an alkyl ester of acrylic acid, copolymeric resin of acrylonitrile, butadiene and styrerie, referred to as an ABS resin hereinafter, or chlorinated polyethylene either alone or in combination. Although improvements can be obtained in the impact strength of the articles molded from such a resin composition, no improvement can be obtained in the flowability behavior of the resin composition so that the resin composition is unsatisfactory with respect to the moldability and smoothness of the surface of the molded articles. Similar disclosures are given in Japanese Patent Publication No. 48-57846 and Japanese Patent Kokai No. 60-179443.